. >> the brothers are also suspected of killing a massachusetts institute of technology police officer during their spree. >> authorities are also focusing their investigation on the tweets sent out by dzhokar tsarnaev before and after the bombing. our tech reporter gabe slate has that part of our coverage. >> let us take a look at this twitter a county only one mention the boston marathon on august 2012. he was responding to somebody else's reference. he says that boston marathon is not a good place to smoke. we cannot see the first part of the conversation we do not know the context if it is relevant or not. and on april 7th, one week before the bombing he said that if you have the knowledge and the inspiration the only action you have is to take action. this was about five actors hours and he followed with only just one half of the conversation and the victims of tragedy? el a o l, those people are and he said that there are people that know the truth and stay silent. 24 hours later, on tuesday he said that everybody wanted to talk like they have something to say but nothing comes o